Vapor phase polysulphide liquid pulping of lignocellulosic materials

ABSTRACT

Impregnating lignocellulosic material, previously saturated with water, in a first stage with an alkaline polysulphide cooking liquor, removing excess liquor, and then cooking the impregnated material in a second stage by rapidly heating with steam.

United States Patent Clayton et al.

[54] VAPOR PHASE POLYSULPHIDE LIQUID PULPING OF LIGNOCELLULOSICMATERIALS [72] Inventors: David W. Clayton, Hudson, Quebec; Asahi Sakai,St.-Laurent, Quebec, both of Canada Pulp and Paper Research Institute ofCanada, Pointe Claire, Quebec, Canada [73] Assignee:

[ Notice: The portion of the term of this patent subsequent to May 9,1989, has been disclaimed.

[22] Filed: Dec. 9, 1969 21 Appl. No.: 883,475

[52] US. Cl ..162/19, 23/48, 23/49, 23/134, 162/68, 162/82 [51] Int. Cl..D21c 3/26 [58] Field of Search ..162/19, 68, 82, DIG. 2, 84; 23/134,49, 48

[451 *May 23, 1972 FOREIGN PATENTS OR APPLICATIONS 7,501 1963 JapanPrimary ExaminerS. Leon Bashore Assistant ExaminerArthur L. CorbinAttorneyStevens, Davis, Miller & Mosher [57] ABSTRACT lmpregnatinglignocellulosic material, previously saturated with water, in a firststage with an alkaline polysulphide cooking liquor, removing excessliquor, and then cooking the impregnated material in a second stage byrapidly heating with steam.

9 Claims, 2 Drawing Figures Patented May 23, 1972 2 Sheets-Sheet l KAPPANUMBER INVENTORS DAVID w. CLAYTON,

ASAHl SAKAI M, wmwh ATTORNEYS Patented May 23, 1972 2 Sheets-Sheet 2 ATKAPPA No.30

30V 0 5; AJDQ 52.0%

KP-A

POLYSULPHIDE CONSUMED. ON WOOD INVENTORS 8 ha u N N R 0 0 W in A MA LA Cwm A W8 VA A D VAPOR PHASE POLYSULPHIDE LIQUID PULPING OFLIGNOCELLULOSIC MATERIALS The present invention relates to theproduction of pulp in improved yield from lignocellulosic materials suchas wood, straw and grasses by a vapor phase polysulphide process. Inparticular the present invention relates to the production of cellulosicpulp in increased yield by a modification of the conventional liquidpolysulphide process in which process the amount of polysulphide andalkali required is substantially reduced.

In the conventional kraft or sulphate process for the production ofcellulosic pulp, in particular wood pulp, the subdivided wood or otherlignocellulosic material is cooked in an aqueous solution of sodiumhydroxide and sodium sulphide at a temperature of the order of 170 C.for the time required to produce a pulp in the required yield. Theresulting pulp contains lignin and carbohydrate in a ratio which isdetennined by the specific conditions of the pulping process i.e. thetemperature cycle, the time, the liquor-to-wood ratio and the ratio ofchemical to wood. In general the ratio of carbohydrate to lignin in thepulp from a given material at a given yield is a quantity which varieswithin very narrow limits being virtually fixed by the nature of theprocess.

In the conventional single stage polysulphide process the aforesaidcooking is effected in the presence of polysulphide ion suitablyprovided by adding sodium polysulphide or elemental sulphur to thecooking liquor and the pulping operation is carried out underappropriately modified conditions which are well known to those skilledin the art. The polysulphide pulps which are obtained have a higherratio of carbohydrate to lignin than the pulps obtained in theconventional kraft or sulphate process at the same yield, due to thestabilization of the wood polysaccharides against alkaline degradationby the polysulphide ion. However, while the product pulps of theaforesaid single stage polysulphide process compare favorably with thepulps obtained in the conventional kraft or sulphate process and inparticular pulps produced by the polysulphide process are usually darkerin color than kraft pulps with the same Kappa number, and have easierbeatability, higher strength in burst and tensile, slightly lowerstrength in double fold and tear and have similar bleachability, theconventional single stage polysulphide process is subject to severalproblems inter alia based upon the tendency of the polysulphide ion todecompose in the presence of hydroxyl ions to form hydrosulphide andthiosulphate ions. The rate of decomposition is increased appreciablywhen the temperature exceeds 100 C. and in a conventional single stagepolysulphide process most of the polysulphide is destroyed before themaximum cooking temperature is reached. Thus, due to the tendency of thepolysulphide ion to decompose in the presence of the hydroxyl ion it isfound that for a given Kappa number the percentage of polysulphide ionconsumed with increase in yield of the pulp rises steeply, with theresult that the sulphur loading on the recovery system of the mill isexcessive and extremely difficult and expensive to handle. As a resultthe single stage conventional polysulphide process has not met withsubstantial commercial success.

In an attempt to overcome the aforesaid problem of decomposition of thepolysulphide ion and its attendant increase in the amount of alkaliconsumed in the single stage polysulphide process, it is proposed inJapanese Pat. No. 7,501 (1963) to effect the process in two stages,involving the treatment with polysulphide in the absence of alkali at atemperature of about 130 C. over a period of an hour and to stabilizethe polysaccharides against degradation in the lignocellulosic materialand subsequently after removal of the excess polysulphide liquor, whichliquor, as it contains residual polysulphide ion, can be reused in theprocess, cooking of the stabilized lignocellulosic material to 170 C.for about an hour with sodium hydroxide or kraft liquor with a 15percent sulphidity to obtain the required pulp. Whilst the two stageprocess of the Japanese patent gives yield increases of 5 to 7 percentbased on wood when compared with kraft pulps of the same lignin content,the process of the Japanese patent still has the disadvantage that asubstantial quantity of the polysulphide is destroyed in thestabilization stage without it reacting with the polysaccharides in thelignocellulosic material. As such, the percentage of polysulphideconsumed for a given increase in yield of the pulp at a constant Kappanumber is still high and the recovery system of a conventional kraftpulp mill would find it expensive and difficult to handle the sulphurloading.

In a further improvement of the polysulphide process as set forth in ourco-pending U.S. application Ser. No. 665,710 filed Sept. 6, 1967, theproduction of pulp from lignocellulosic material by the polysulphideprocess is effected in three stages in which the decomposition of thepolysulphide during the impregnation of the polysaccharides in thelignocellulosic material is substantially reduced and at the same timean increase in pulp yield is obtained at essentially the same Kappanumber over that obtained by the conventional kraft or single stagepolysulphide process. Thus, in accordance with the invention of U.S.application Ser. No. 665,710 it was found that when the lignocellulosicmaterial in subdivided form is subjected to impregnation in a separatestage from the stabilization stage, and in the impregnation stage thetemperature is maintained sufficiently low so that essentially nopolysulphide ion decomposes and the excess of polysulphide liquorcontaining no added sodium hydroxide is removed from the materialbetween impregnation and stabilization stages, it is possible to effectboth the impregnation and stabilization stages under conditions whichare optimum for both impregnation and stabilization and thus avoid theproblem which previously occurred, particularly in the process of theJapanese patent, namely that the conditions for stabilization aredirectly opposed to the conditions for desirable impregnation. Inparticular in US. patent application Ser. No. 665,710 there is provideda process for the production of cellulosic pulp for lignocellulosicmaterial which comprises impregnating said material in subdivided forme.g. as chips, with a polysulphide liquor containing no added sodiumhydroxide at a temperature below that at which substantial decompositionof the polysulphide occurs, removing excess polysulphide liquor from theimpregnated materials, stabilizing the impregnated materials againstalkaline degradation by increasing the temperature of said materials andsubsequently delignifying said stabilized materials by cooking saidmaterials in a cooking liquor containing sodium hydroxide. In practice,in the process of the aforesaid U.S. application Ser. No. 665,710 theimpregnation is effected at a temperature not exceeding 1 10 C. andusually in the range of to C., the stabilization is effected at atemperature not less than C. usually under superatmospheric pressure andthe stabilized material is cooked at a temperature in the range to C.Thus by subdividing the first stage into indivudial stages forimpregnation and stabilization it is possible to obtain optimumconditions for stabilization without the necessity of protecting thepolysulphide against decomposition because the excess of polysulphide iswithdrawn and the impregnation is also carried out under conditions inwhich the least loss of polysulphide occurs. This process, which iseffected in three stages in which the decomposition of the polysulphideduring impregnation of the polysaccharides in the lignocellulosicmaterial is substantially reduced, produces an increase in pulp yieldover that obtained by the conventional Kraft, single stage polysulphideprocess and the two stage polysulphide process of the Japanese patentwhen the pulp yields are compared at substantially the same Kappanumber.

In the three stage polysulphide process of U.S. application Ser. No.665,710 the polysulphide solution in the impregnation stage contains noadded sodium hydroxide and the polysulphide solution has a pH below 12.5which avoids substantial decomposition of the polysulphide ions intothiosulphate and hydrosulphide ions, the rate of which decompositionbecomes appreciable above a pH of about 12.5. In a further improvementof the three stage process of U.S. application Ser. No. 665,710 it wasfound that it was also desirable to maintain the pH in the impregnationstage above about 1 1.5 to avoid conversions of the polysulphide ions inthe impregnation liquor into sulphur and hydrosulphide ions and this iseffected according to the process as disclosed in our co-pending U.S.application Ser. No. 864,740 by the addition of ammonium hydroxide tothe impregnation stage, and in the subsequent stabilization stage afterremoval of the impregnation liquor, the impregnated lignocellulosicmaterial has its temperature raised above 130 C., suitably by steaming,with addition if necessary of further ammonia to provide optimumstabilization conditions. This is followed by the delignification stagewhich involves the addition of sodium hydroxide or kraft liquor in asimilar manner as in the process of U.S. application Ser. No. 665,710

According to the invention of our co-pending U.S. application Serial No.864,740 there is provided in a process for production of a cellulosicpulp from lignocellulosic material which comprises substantiallycompletely impregnating said material in subdivided form with apolysulphide liquor containing no added sodium hydroxide and at a pHbelow 12.5 and at a temperature below that at which substantialdecomposition of the polysulphide occurs, removing the excess ofpolysulphide liquor from the impregnated material, stabilizing theimpregnated material against alkaline degradation by increasing thetemperature of said material and subsequently delignifying thestabilized material by cooking said material in a cooking liquorcontaining sodium hydroxide, the improvement which comprises effectingimpregnation of the material for a polysulphide liquor containingammonium hydroxide in an amount sufiicient to maintain a pH of at leastH5 and effecting stabilization by heating the impregnated material inthe presence of ammonia gas under superatmospheric pressure. In thisprocess it has been found that a still further increase in pulp yield isobtained at essentially the same Kappa number over that obtained by theconventional Kraft, single stage polysulphide process, two stagepolvsulphide process of the Japanese patent and three stage polysulphideprocess of U.S. application Ser. No. 665,710 for a particularpolysulphide consumption and corresponding alkali consumption.

In the aforesaid process of U.S. applications Ser. Nos. 665,710 and864,740 the impregnation of the chips with the liquor is effected oneither dry chips or water-saturated chips, the use of water-saturatedchips being preferred as the impregnation of wet chips can be effectedsubstantially at atmospheric pressure whereas with dry chips to obtainuniform impregnation within a reasonable period of time one has to usepressure which may be inconvenient.

It has now been found according to the present invention that there isan alternative process to the aforesaid three stage processes of US.applications Ser. Nos. 665,710 and 864,740 and in particular whenmoisture saturated chips are impregnated with a conventionalpolysulphide liquor such as that used in the single stage polysulphideprocess at a temperature below 95 C. and after removal of the excess ofliquor the impregnated chips are subjected to cooking at elevatedtemperatures in the vapor phase at an elevated temperature preferably inthe range 170 to 185 C., that the pulp yield obtained at constant Kappanumber for a particular polysulphide consumption, particularly below 6percent on wood, is superior to that obtained in the single stagepolysulphide process, below about 4 percent polysulphide consumption onwood is superior to that obtained in the three stage process of U.S.application Ser. No. 665,710 and below about 2.5 percent on wood issubstantially superior to that obtained by the process of U.S.application Ser. No. 864,740. According to the present inventiontherefore there is provided in a process for the production ofcellulosic pulp from lignocellulosic material which comprises heatingsaid material in subdivided form with an alkaline cooking liquorcontaining polysulphide ion, the improvement which comprises in a firststage impregnating said subdivided material previously saturated withwater, with said cooking liquor at a temperature below that at whichsubstantial decomposition of the polysulphide ions occurs, and removingexcess liquor from said impregnated material and in a second stagecooking said impregnated material at elevated temperature in the vaporphase.

Thus, in the aforesaid three stage polysulphide processes of U.S.application Ser. Nos. 665,710 and 864,740 the processes inherentlyinvolve protection of the polysulphide in the impregnation stage,complete and uniform impregnation into the wood, promotion of thestabilization reaction on the polysaccharides in the wood, suppressionof degradation of the polysaccharides during the stabilization stage,promotion of delignification, avoidance of hydrolysis of thepolysaccharides. However, in the aforesaid three stage processes, in thefinal cooking stage and possibly also in the stabilization stage thewood chips are heated with hot alkali, e.g. sodium hydroxide containingpolvsulphide ions, and it is believed that in such processes thehemicelluloses in the wood are readily dissolved by the alkalinesolution at such high temperature and are therefore lost, even thoughthe reducing end groups of the hemicelluloses are stabilized against thealkaline peeling reaction by the presence of the polysulphide ion. It isthus believed that by operating with conditions which avoid the heatingof the wood chips with a large volume of strong alkali solution,particularly heating more than twice with such solution, the dissolutionof the hemicelluloses is reduced to a minimum and thus the process ofthe present invention, in addition to the above advantages to beobtained in the aforesaid three stage process, has the additionaladvantage of preventing dissolution of wood hemicelluloses and thereforeincreasing the total pulp yield for a particular polysulphideconsumption. However, it is also found that an application of a lowliquor-to-wood ratio in the stabilization stage also helps to reduce theleaching out of the impregnated polysulphide from the wood chips andthus helps to maintain the highest possible concentration ofpolysulphide in the chips at a given polysulphide charge. A lowliquor-to-wood ratio is of course inherent in a vapor phase cooking ofthe impregnated chips.

Thus the process of the present invention is essentially a two stageprocess, the first stage of which is an impregnation stage in which thewood chips are impregnated with the polysulphide cooking liquor, whichis essentially an aqueous solution of sodium hydroxide e.g. kraft liquorcontaining polysulphide ions, suitably obtained by the addition ofsodium polysulphide to the liquor. The impregnation stage desirablyeffected at a temperature below 95 C. and suitably in the range of 70 toC., the maximum temperature of C. being a lower temperature than thatC.) specified as maximum for the three stage polysulphide pulpingprocess, because the decomposition of the polysulphide ion in thepresence of an excess of sodium hydroxide is much more rapid than for asolution of polysulphide alone, since the latter can be heated to 110 C.without undue decomposition.

To achieve the advantages of the present invention it is essential thatthe wood chips be fully saturated in order to obtain a uniformimpregnation of the chips and to obtain a sufficient charge ofpolysulphide in the chips. The saturated moisture content of the chipsmay be obtained by merely soaking the chips in warm water for asufficient time. The impregnation of the wet chips with the polysulphideliquor allows the impregnation to be effected substantially atatmospheric pressure as opposed to superatmospheric pressure with drychips. As used herein the term water saturated as applied to thelignocellulosic material refers to subdivided material, e.g. chips inwhich at least the centers thereof are saturated with water. Forsoftwoods the water content is usually in the range 180 percent to 200percent by weight and for hardwoods the water content is in excess ofpercent by weight. The saturated wood chips generally have a moisturecontent in excess of about percent by weight. The impregnation of thewet chips is suitably effected for a period of the order of 9s an hourto an hour as it is found that in this range of time the wet chips canbe impregnated with of the quantity of the chemical which can beintroduced after 4 days. Further after A an hour The cooked chips werethen disintegrated and the pulp was screened on a vibrating flat screenwith 0.010 inch slits. The .weights of the screen rejects and thescreened d pregnated charge containing polvsulphide sul to thepercentages, based on ovendry wood 1.

lncooksS-192, 196,193,250, 251,197,171,172,173 and 174 the impregnatedchips were then steamed for 15 minutes at a temperature of 100 C. andwere subsequently steamed at a temperature of 175 C. for 90 minutes at115 psig. In cooks 8-106, 107, 108 and 109, the impregnated chips wereraised directlv to a temperature of 175 C. over 60 minutes and were 10maintained at that temperature for 90 minutes by steaming at a pressureof 1 15 psig.

etermined and these were summed up to provide the total yield. Theconditions and results are given in Table 1 followmg.

the unimpregnated area in the wet chips is found to have disappeared incross section thereof and accordingly where a sufficient amount ofchemicals can be introduced within a short time by means of a highconcentration the size of the impregnation vessel can be reduced becausethe throughput is higher.

The concentration of the polysulphide sulphur in the liquor is suitablyin the range up to 50 grams per liter, for above about 50 grams perliter no substantial advantage is gained.

The liquor-to-wood ratio in the impregnation stage is suitably withinthe range 2.5 to 1 to 4 to l, as above the ratio of 4 to 1 an infiniteeffect is approached, where a two fold increase in the ratio leads onlyto a 10 percent increase in impregnation under the same conditions. Ahigh liquor volume places a great load on the circulation system andinherently exposes a large quantity of polysulphides to the risk ofdecomposition in a given time. Accordingly to obtain efficientimpregnation it is preferable to keep the liquor-to-wood ratio as low aspossible and to increase the polysulphide concentration. As aforesaid adesirable impregnation is below 6 percent by weight preferably below 4percent by weight and more preferably below 2 percent by weight onovendry wood, which gives optimum increases in yield over the aforesaidthree stage polysulphide processes and over the single conventionalpolysulphide process.

Subsequent to the impregnation of the wood chips, which from a practicalpoint of view must be as complete and uniform as possible in order toproduce a useful pulp, the excess of liquor is removed from contact withthe chips and may be recycled for further use in impregnation of freshchips. The impregnated chips are then subjected to a vapor phase cookingprocedure in which the temperature of the chips is raised, suitably bysteaming, to a temperature in the range 170 to 185 C. and preferablv inthe range 170 C. to 175C.

In a particular embodiment of the invention, low pressure steaming afterimpregnation of the chips and before the vapor phase cooking of thechips is desirably effected, as this operation ensures that thestabilization reaction is complete before delignification takes place.The vapor phase cooking of the chips is suitably effected at a cookingpressure in the range 115 to 150 psig, suitably for a period in therange to 90 minutes.

The present invention will be further illustrated by way of thefollowing examples:

EXAMPLE 1 Black spruce guillotine chips with a size of 1 inch by (graindirection) by l/6 inch thick were prepared from black spruce logs. Thechips were air-dried at atmospheric pressure to a moisture content of 10percent based on dry wood and were separated into aliquots of 326 grams(ovendry weight). Each aliquot was saturated with water in a pressurevessel by two steam purges at 30 psig for 2 minutes each, followed byforced impregnation with water at 70 C. under a pressure of nitrogen gasat 100 psig for 1 hour. Following this the chips were sealed in apolyethylene bag and were stored in a cold room at 3 to 5 C. for from 2to 5 weeks. Before the chips were subjected to cooking, any free waterwas drained away by keeping the wet chips in a wire mesh basket (60mesh) for 1 hour. At the point when the surface of the chips wasstarting to dry the moisture content of the chips was l79fl percent(based on dry wood), which approached the water saturation point forspruce wood (190 to 200 percent). In a series of cooks according to thepresent invention a number of these aliquots were subjected in animpregnation stage to heating with 1,305 ml. of an aqueous kraft liquorcontaining sodium tetrasulphide at a liquor-to-wood ratio of 4 to l. Theimpregnated chips were in each case heated to 90 C. over a period of 15minutes in a 2.5 liter bomb and were maintained at this temperature atatmospheric pressure for 60 minutes. The effective alkali, sulphidity,sodium hydroxide content, sodium monosulphide, and sodium tetrasulphidecontent of the liquor are set for each cook in Table l. The excess ofpolysulphide liquor was then removed, leaving an im- The concentrationof the polysulphide in the fresh and withdrawn liquors was determined bythe acidimetric method proposed by K. Johnsen, (Norsk Skogind 20, No.3:91 95, Mar., 1966) and improved by P. Ahlgren, (Svensk Papperstidn. 21No. 152730-733, Nov. 15, 1967). From the above results graphs wereplotted as shown in the accompanying drawings in which:

FIG. 1 shows plots of Kappa number against total pulp vield at differentpolysulphide consumptions. The figures in parenthesis by each curve arepolysulphide sulphur consumptions, expressed as percentages based onovendry wood.

FIG. 2 shows plots of total pulp yield against polysulphide consumptionat Kappa number 30 for the process of the present invention identifiedas II-P, as well as for the process of U.S. application Ser. No. 864,740identified as III-Q, U.S. application Ser. No. 665,710 shown as III-Eand a conventional single stage polysulphide process shown as l-C.

In FIG. 2 the plots identified as III-Q, III-E and I-C are taken.

directly from FIG. 2 in U.S. application Ser. No. 864,740, which graphsare based upon the results therein and the results in U.S. applicationSer. No. 665,710.

It will be seen from the graphs that below a polysulphide consumption ofabout 2.5 percent by weight, the two-stage 1. In a process for theproduction of pulp from a lignocellulosic material which comprisesheating said material in chip form with an alkaline cooking liquorcontaining polysulphide ion the improvement which comprises in a firststage impregnating said lignocellulosic material in chip form previouslysaturated with water with said cooking liquor at a temperature below 95C. and removing the excess of liquor from said impregnated material, andin a second stage cooking said impregnated material with saidimpregnating polysulphide liquor by rapidly heating to an elevatedtemperature with steam.

2. A process as claimed in claim 1 in which the polysulphide consumptionon wood is up to 6 percent.

3. A process as claimed in claim 1 on which the polysulphide consumptionon wood is less than 4 percent.

4. A process as claimed in claim 1 in which the polysulphide consumptionon wood is less than 2.5 percent.

5. A process as claimed in claim 1 in which the impregnated material iscooked in the vapor phase by steaming, under superatmospheric pressure.

6. A process as claimed in claim 1 in which the impregnation temperatureis in the range 70 to 90 C.

7. A process as claimed in claim 1 in which the cooking temvapor phasepolysulphide pulping process (II-P) of the present 2 5 perature 1S m therange 175 to 185 invention gives higher pulp yields, at the same Kappanumber- (FIG. 2) for the same polysulphide consumption, than theprocesses III-Q, III-E and LC; for a polysulphide consumption of up toabout 4 percent by weight, it is better than processes Ill-E and LC, andbetter than process I-C for a polysulphide consumption below about 6percent by weight.

We claim:

8. A process as claimed in claim 7 in which the cooking pressure is from1 15 to psig.

9. A process as claimed in claim 1 in which the impregnated chips aresteamed at a temperature of about 100 C. to effect stabilization and thetemperature subsequently raised by steaming to be in the range to C. toeffect delignification.

2. A process as claimed in claim 1 in which the polysulphide consumptionon wood is up to 6 percent.
 3. A process as claimed in claim 1 on whichthe polysulphide consumption on wood is less than 4 percent.
 4. Aprocess as claimed in claim 1 in which the polysulphide consumption onwood is less than 2.5 percent.
 5. A process as claimed in claim 1 inwhich the impregnated material is cooked in the vapor phase by steaming,under superatmospheric pressure.
 6. A process as claimed in claim 1 inwhich the impregnation temperature is in the range 70* to 90* C.
 7. Aprocess as claimed in claim 1 in which the cooking temperature is in therange 175* to 185* C.
 8. A process as claimed in claim 7 in which thecooking pressure is from 115 to 150 psig.
 9. A process as claimed inclaim 1 in which the impregnated chips are steamed at a temperature ofabout 100* C. to effect stabilization and the temperature subsequentlyraised by steaming to be in the range 175* to 185* C. to effectdelignification.